1. Field of the Invention
This invention relates to a solid oxide fuel cell stack which utilizes metallic foils as interconnects, thereby eliminating the need for glass seals used in conventional solid oxide fuel cell stack systems. In addition to eliminating the need for glass seals between fuel cell units of the fuel cell stack, the stacks can be subjected to rapid variations in temperature without cracking, and thermal expansion match between components is not required in contrast to known solid oxide fuel cell stack designs.
2. Description of Prior Art
Fuel cell systems are known and used for the direct production of electricity from standard fuel materials including fossil fuels, hydrogen, and the like. Fuel cells typically include a porous anode, a porous cathode, and a solid or liquid electrolyte therebetween. Fuel materials are directed along and in contact with the anode of the fuel cell system, while an oxidizing gas, for example air or oxygen, is allowed to pass along and in contact with the cathode of the system. As a result, the fuel is oxidized, with the oxidizing gas being reduced in order to generate electricity. The electrolyte is designed to allow charge transfer between the anode and the cathode.
Solid oxide fuel cells have attracted considerable attention as the fuel cells of the third generation following phosphoric acid fuel cells and molten carbonate fuel cells of the first and second generations, respectively. Solid oxide fuel cells have an advantage in enhancing efficiency of generation of electricity, including waste heat management, with their operation at high temperature, above about 650.degree. C. However, because a single fuel cell unit only produces an open circuit voltage of about one volt and each cell is subject to electrode activation polarization losses, electrical resistance losses, and ion mobility resistant losses which reduce its output to even lower voltages at a useful current, a fuel cell stack comprising a plurality of fuel cell units electrically connected to each other to produce the desired voltage or current is required. Planar solid oxide fuel cell stacks typically comprise a plurality of stacked cathode-electrode-anode-interconnect repeat units. Channels for gas flow, either in a cross-flow or a co-flow or a counterflow configuration, are usually incorporated into the interconnect. In order to permit the transport of gases through the channels, known interconnects are usually at least 1.5 to 2 mm in thickness. As a consequence, both the cell and the interconnect, whether of ceramic or metallic material, are rigid. As a result, to achieve an effective seal, the mating surfaces between the cell and the interconnect must be flat and parallel. In addition, because all of the components are rigid, even with good flatness, it is usually necessary to use a glass material for sealing. Solid oxide fuel cell systems are taught, for example, by U.S. Pat. No. 5,238,754 to Yasuo et al.; U.S. Pat. No. 5,258,240 to Di Croce et al.; U.S. Pat. No. 4,761,349 to McPheeters et al.; and Re. 34,213 to Hsu.